This invention concerns particulate media used in a bed for the purification of water or other liquids.
It has been known for some time that solids can be removed from a liquid by passing the liquid through a bed of particulate media. Examples of such methods are described in U.S. Pat. Nos. 4,608,181 (Hsiung et al.), 4,725,367 (McKim et al.), 4,780,219 (Witek), and 5,009,776 (Banks).
Heretofore, the media has been selected, for convenience, from among commercially available products intended for other uses. For example, some filters have been made using polypropylene or polyethylene pellets of the type that are sold to supply injection molding machines. While such prior media have worked, they are not optimized for operation in liquid purification.
Alternatively, water purification apparatuses have employed beds of plastic chunks which are made by comminuting larger bodies of a plastic material. For example, media has been made by melting polypropylene beads to form blocks of polypropylene which are subsequently cooled and ground up. The resulting polypropylene chunks had the properties described in U.S. Pat. No. 4,608,181 (Hsiung et al.). While plastic chunks can be manufactured to have excellent properties for liquid purification, they are expensive to make because energy must be consumed to melt the plastic material to form the large bodies. And, as a result of the heating, portions of the material may be altered undesireably in composition or properties. Also, because such chunks are made using a grinding process, plastic fines are produced. These fines must be removed before the media chunks can be used in a liquid treatment vessel. If the media is not cleaned properly, the fines will clog screens and other apparatus thereby causing premature headloss. Cleaning to remove fines is typically conducted in the field, so there is also a problem with environmentally proper disposal of the waste plastic fines.
Thus, there is a need for a particulate media that is produced affordably and yet provides excellent results when used for liquid purification.
It has now been found that a particulate media, particularly suitable for use in purifying water and other liquids, can be made from common starting materials by a simple method. The resulting media performs as well or better than the best prior media and is produced at a fraction of the cost of other high-efficiency media.
Media of the present invention are beads which can be made from a common polymeric precursor material. In particular, the beads can be made from commonly available plastic pellets which, as delivered, are of a suitable volume and are otherwise suitable for use in liquid treatment, but which pellets are not be of the best shape or texture. The composition of the polymeric material is selected so that the media will be suitable for its intended use, e.g. will have a desired specific gravity. For beads used for the preparation of potable water, the selected material should be nontoxic. The polymeric material is also selected for its ability to be cold worked.
To roughen the surfaces of such polymeric pellets, the pellets are passed through a scarifier or like apparatus. A bed of media with such scarified surfaces will retain a greater amount of particulate material than a bed of similar media with unroughened surfaces. The use of such media allows a treatment apparatus to operate for a longer period of time between bed cleanings.
Also, to increase their surface area, plastic pellets are compressed into disc-like shapes. After compression, a resulting media bead has generally convex upper and lower surfaces and a perimeter or annular edge surface on the rim of the disc. Beads of a such a disclike shape have a greater surface area and should pack more densely than uncompressed pellets. Disc-like beads are thus better in collecting solids from a passing liquid.
To still further increase the surface area of a solids separation bed, multiple grooves are provided in the surfaces of the media beads. Such grooves can be inscribed by passing plastic pellets through a roller mill having rolls with striated surfaces. The pattern of closely spaced grooves on the roll surfaces is transferred to the surfaces of the plastic pellets. Preferably the grooves are formed in the pellets, at the same time the pellets are flattened, by a single pass through a roller mill.
It is among the advantages of the present invention that the beads have excellent properties for use in a liquid purification bed and that they are produced at low cost, with little waste, and with no environmental problems. It is further advantageous that beads of such high quality are ready for use immediately upon their manufacture, without cleaning.